The invention relates to a method for shifting gears in a dual clutch transmission within a motor vehicle comprising a first clutch and a second clutch as well as a first partial transmission and a second partial transmission, wherein the first partial transmission comprises the first clutch and a first group of gears and the second partial transmission comprises the second clutch and a second group of gears.
For dual clutch transmission shifting or changing gears is typically conducted by two types of methods: In the pull-up and push-back shifting at first a target gear is engaged in a second partial transmission that is not supposed to transmit any torque prior to shifting. Thereafter, the applied motor torque is transmitted according to a predetermined motor load from the first clutch to the second clutch. During this action, the second, passive clutch is controlled under slippage such that the rotational speed of the motor equals to the rotational speed of an input shaft of the first partial transmission. Subsequently, the rotational speed of the motor is controlled to rotate at the rotational speed of an input shaft of the second partial transmission by reducing of the motor torque and controlling the torque transmitting capacity of the second clutch. Thereafter, the second clutch can be closed completely. Such a control of the torque transmitting capacity of the first and second clutches as well as the subsequent adaption of the rotational speed of the motor to the rotational speed of the second input shaft of the second partial transmission is disclosed in the EP 1 507 103 B1.
For the pull-back and push-up shifting the sequence of the torque transmissions steps (decreasing of the torque transmitting capacity of the first clutch while simultaneously increasing the torque transmitting capacity of the second clutch) is reversed: At first, the first clutch is brought into a slippage operational mode and the rotational speed of the motor is increased to the rotational speed of the input shaft of the second partial transmission. Prior to this action and during this action, the target gear is engaged in the second partial transmission. As soon as the rotational speed of the motor equals to the rotational speed of the input shaft of the second partial transmission, the torque is transferred from the first clutch (that is still active) to the second clutch (up to now passive clutch) and the shifting process is terminated.
These shifting methods have two limitations making these appear disadvantageous during the starting process of the vehicle: On the one hand, these methods work best when the rotational speed of the motor is between the rotational speeds of the two input shafts, on the other hand, this shifting method is very sensitive against changing motor torques or motor loads (for example due to changing acceleration pedal positions).
During the starting process, the rotational speed of the motor is typically above the rotational speed of the two input shafts of the partial transmission wherein typically a first forward gear on the one hand and a second forward gear of the vehicle on the other hand are preselected. Moreover, for reasons of comfort, the shifting process should stretch over a longer time period and is therefore in particular sensitive against changing gas pedal positions that occur in particular in a low speed and low load range during the starting process. Further, in the method as described above, no gear shifting can be conducted when the rotational speed of the input shaft of the second partial transmission is below the idle rotational speed of the motor. This prevents shifting into higher gears at an early stage that is desirable in case of low acceleration pedal positions due to the typical big transmission steps.